Fuel cell stacks have been known to include bipolar plates to collect and distribute electrons between fuel cells. The bipolar plates may be made from composite materials or metal substrates that have been machined or from relatively thin metal substrates that may be stamped to provide reactant gas flow fields and coolant fluid flow fields. Such metal bipolar plates may be coated or uncoated.
Metal bipolar plates are subject to corrosion during the operation of the fuel cell, mainly because of the fluoride ions released as a result of membrane degradation. For example, metal dissolution of anode plates can result in the release of Fe, Cr, Ni ions which may impair proton conductivity of the fuel cell membrane. Cathode bipolar plates are often covered with a thin passivation oxide. However, such oxide creates a surface with high electrical contact resistance.
Protective coatings such as Ti, Ta, Cr and the like have been used for non-precious metal corrosion protection. These metals/alloys maintain corrosion resistance because of the development of protective passive oxide layers on their surfaces. Again, these passive oxides are generally non-conductive and therefore produce a high electrical contact resistance surface. Some of these metals/alloys may also undergo localized corrosion under specific fuel cell operation conditions. Precious noble metal coatings have been utilized as solutions to these problems but such coatings are prohibitively expensive for commercial applications.
For water management, it is desirable for metal bipolar plates to have a low contact angle at the bipolar plate/water border, that is, a contact angle less than 40°. However, the contact angles for precious metal/water interface are typically larger than 40°. Titanium nitride coatings have been proposed as a corrosion-resistant plating for bipolar plates. Although titanium nitride coatings are cost-effective, such coatings do not provide satisfactory protection for the bipolar plate material. Further, titanium nitride coatings develop relatively low water affinity with a contact angle close to 60°.